A plan by the interim Labour leader Harriet Harman to give half the places in the shadow cabinet to women has been knocked back by the party's MPs, who voted to make the proportion a third.

Labour MPs also defeated a move to end the practice of holding an election to the majority of shadow cabinet posts, instead of the positions being appointed by the party leader.

However, the shadow cabinet vote will now take place every two years rather than annually, a slight diminution of the parliamentary party's power but one unlikely to represent much of a transfer of power to the leadership.

In a series of votes on how to reform the party while it is in opposition, Labour MPs declined to reduce the size of the shadow cabinet to 23 places from the present 26 – larger than that of the government's cabinet of ministers by three.

The defeat of gender parity was described by shadow work and pensions secretary Yvette Cooper as "disappointing".

Harman had called for a 50:50 split in the early days of her tenure as acting leader but with time softened her position.

Instead, she proposed the initial proportion should reflect that of the parliamentary party at large — 31.5% — but rise to 50% over the course of this parliament.

In the event, MPs opted only for a quota of 31.5% with no possibility of the compulsory ratio increasing with time. MPs had disliked the proposal, saying men would be forced to contemplate vacating their shadow cabinet portfolios to make way for women over a period of time. An alternative vote system was used, and while equal numbers supported the 50:50 option and the 31.5% option, in the second round more MPs opted for the lower quota.

The result, according to Cooper, means there will be at least eight women in the shadow cabinet of 26, including Harman as deputy leader of the party and Jan Royall as shadow leader of the Lords.

Even eight woman in the shadow cabinet, Cooper pointed out, would be greater than the number of women on the coalition government's front bench, four.

This evening's result also saw Labour MPs vote to elect their chief whip at the beginning of each parliament instead of the post being appointed by the party leader.

The existing chief whip, Nick Brown, was reported to have been lobbying his colleagues to vote in favour.

It is said Brown has now effectively secured his own power base within the party, nullifying any leader's power to sack him.

Brown was lobbying hard for the reform, with one Labour MP saying he was holding half-hour meetings with undecided MPs.

Colleagues in the whips' office had also been deployed to persuade MPs, and even the former whip and newly ennobled peer Tommy McAvoy returned to the Commons to help.

Brown is a close confidante of Gordon Brown, and some MPs interpreted Nick Brown's intense campaign as a potential problem for any future leader, fearing it could mean the former prime minister maintaining a power base under a new regime.

Supporters of David Miliband had mobilised against Nick Brown's efforts; should Miliband become leader, one said they would be likely to stand a candidate against Brown in order for the leader to have a chief whip of his own choice.

While leaders can appoint and sack ministers at will while in office, during times of opposition party rules give MPs the right to vote in the majority of shadow cabinet members, currently 19.

This is not the case for the Conservatives or Liberal Democrats when in opposition. It could mean the new Labour leader having to work with shadow ministers from the opposite wing of the party.

Nominations close on 26 September for the 19 elected members of Labour's shadow cabinet. Results of voting will be announced on 7 October.